The present invention relates to an MR (Magnetic Resonance) imaging method, a 3D (Three-Dimension) imaging method and an MRI (Magnetic Resonance Imaging) system, and more specifically to an MR imaging method, a 3D imaging method, and an MRI system, each of which is capable of suppressing the adverse effect of residual magnetization caused by an encode order.
FIG. 1 is a diagram showing one example of a pulse sequence for a 3D scan, which is executed in an MRI system.
In the pulse sequence, an echo is sampled while an imaging region is being excited by an RF pulse R and a Z-direction phase axis pulse SL, encoding in a YDZ space is being performed by a Y-direction phase axis pulse PEy and a Z-direction phase axis pulse PEz, and frequency encoding is being carried out in an X direction by a read-axis pulse RD. Finally, rewinding in a YZ space is carried out by a Y-direction phase axis pulse RWy and a Z-direction phase axis pulse RWz. This is repeated while encoding in the YZ space is being changed in order so as to fill a YZ-K space. Tr indicates a repetition time.
Loci at data points on a K space, which are caused by the order in which the encoding in the YZ space is changed, are called trajectories. In general, the Elliptical Centric View Ordering has been adapted for the trajectories in the YZ space at the 3D scan. The “Elliptical Centric View Ordering” is hereinafter represented as “ECVO”.
FIG. 2 is a conceptual diagram showing an encode order based on the ECVO.
In the ECVO, encoding is carried out in order of decreasing distances from the center Cyz of a YZ-K space. Namely, encode points to which serial numbers “1” through “36” are assigned, are encoded in order of their serial numbers in the YZ-K space shown in FIG. 2 to thereby perform a 3D scan.
Incidentally, the ECVO has been described in “Performance of an Elliptical Centric View Order for Signal Enhancement and Motion Artifact Suppression in Breath-hold Three-Dimensional Gradient Echo Imaging: Alan H. Wilman, Stephen J. Riederer: MRM 38: 793-802(1997)”.
In an MRI system, particularly, a permanent-magnet MRI system, the polarity of residual magnetization of a magnetic material is inverted due to the hysteresis characteristic of the magnetic material each time the polarity of a gradient magnetic field is reversed. This appears on an MR image as an artifact. Thus, the number of times that the polarity of the gradient magnetic field is inverted, may preferably be less reduced.
However, according to the encode order employed in the conventional example shown in FIG. 2, the number of times that the polarity of a gradient magnetic field on or about a Y-direction chase axis is inverted, is 35, and the number of times that the polarity of a gradient magnetic field on or about a Z-direction phase axis exists even 17. Therefore, the number of the polarity inversions increases. Accordingly, a problem arises in that the adverse effect of residual magnetization is considerable.